Field devices are used in industries to control operation of a process such as an oil refinery. A field device, such as a transmitter, is typically part of a process communication loop and is located in the field to measure and transmit a process variable such as pressure, flow or temperature, for example, to control room equipment. A field device such as a valve controller can also be part of the process communication loop and controls position of a valve based upon a control signal received over the process control loop, or generated internally. Other types of controllers control electric motors or solenoids, for example. The control room equipment is also part of the process communication loop such that an operator or computer in the control room is capable of monitoring the process based upon process variables received from transmitters in the field and responsively controlling the process by sending control signals to the appropriate control devices. A process communication loop sometimes includes a portable communicator which is capable of monitoring and transmitting signals on the process communication loop. Typically, such portable communicators are used to configure field devices which form the process communication loop. As used herein, the term “process communications loop” is intended to mean any physical connection and media that carries process signals, regardless of whether the connection forms an actual loop. Thus, a process communication loop can be a HART® or FOUNDATION™ Fieldbus segment, even though such a segment is not strictly considered a loop.
With the advent of low-power microprocessors, field devices have undergone significant changes. Years ago, a field device would simply measure a given process variable, such as temperature, and generate an analog indication in the form of a current varying between 4 and 20 (mA) to indicate the measured temperature. Currently, many field devices employ digital communication technology as well as more sophisticated control and communication techniques. Field devices often employ low-power electronics because in many installations they are still required to run on as little as 4 mA. This design requirement prohibits the use of a number of commercially available microprocessor circuits. However, even low-power microprocessors have allowed a vast array of functions for such field devices.
There has been a dramatic increase in the availability of such microprocessor-based field devices. Such field devices are sometimes termed “smart” or “intelligent.” There has also been a dramatic increase in the availability of software applications that are used to configure, test, and diagnose these smart field devices. Connection of a general purpose computing device, such as a personal computer (PC) or a portable laptop computer is typically accomplished using a modem between the computing device and the intelligent field devices. There is a significant array of process communication protocols such as the HART®, FOUNDATION™ Fieldbus, Modbus®, and Profibus protocols that support the various process control tasks. Moreover, it is common to find multiple communication protocols in use in the very same process installation.
One technique for coupling a general purpose computing device to process communication networks having various process communication protocols is found in U.S. Pat. No. 6,839,790. The '790 patent reports an interface device that includes a re-configurable circuit which provides access to a selected fieldbus network from among several optional fieldbus networks. However, the techniques taught by the '790 patent generally require a user to have a priori knowledge of the particular type of fieldbus to which he or she is connecting. Thus, if a user wants to connect to a Profibus network, the user must make that selection known, and then the interface will reconfigure itself. However, if the user does not know what type of process communication protocol is being used, or if the user's selection is erroneous, the interface may begin communicating using a protocol that is not compatible with the actual protocol in use. This may introduce dangerous signaling levels that may damage, or otherwise degrade communication on the process control loop; damage or otherwise degrade the interface module itself, or potentially interfere with the proper operation of the process control loop.